122 



A HISTORY OF FISHES 



of a Sand Shark (Odontaspis) reveals the fact that there is more 

 than one row of teeth in each jaw, but, whilst those of the row 

 in actual use stand erect on the edge of the jaw, the others lie 

 flat against its inner surface (Figs. 50B; 51 a). These reserves 

 lie in a shallow cavity closed by membrane, the rows packed 

 closely together one upon the other, those of the lower jaw 

 with the points directed downwards, those of the upper jaw 

 in the opposite direction. A closer study of these teeth shows 

 that they are in different stages of development, those of the 

 row nearest to the edge of the jaw being most perfect. As the 



Fig. 51' 



-TEETH OF SHARKS AND RAYS. 



A. Inner view of lower jaw of Blue Shark {Carcharinus dussumieri), X j ; b. Lower 



jaw of Nurse Shark {Ginglymostoma sp.),y.l ; c. Jaws of Guitar-fish (Rhina 



ancylostoma), X ^ ; d. Jaws of Eagle Ray {Myliobatis aquila), X i. 



functional teeth are lost those of the next series move upward, 

 and the whole phalanx is thus constantly marching onward 

 throughout life, a row of teeth doing duty for a time, only to be 

 duly cast off in its turn and replaced by its successor. In certain 

 Sharks and Rays several series of teeth are in use at the same 

 time (Fig. 51B-D), but the replacement takes place in exactly 

 the same manner. 



In size and form the teeth of Sharks exhibit great diversity, 

 ranging from long, slender, awl-like structures to large, flat, 

 triangular teeth (Fig. 52). Not infrequently the teeth of the 

 upper jaw are quite unlike those of the lower, and different 



